Cerebral haemodynamics The cardiac output is about 5 l/min of blood for a resting adult. Blood flow to the brain is about 14% of this, or 700 ml/min. For any part of the body, the blood flow can be calculated using the simple formula Resistance Pressure Blood flow = (2.1) Pressure in the arteries is generated by the heart which pumps blood from its left ventricle into the aorta. (Since pressure was historically measured with a mercury manometer, the units are commonly expressed in terms of [mm Hg], although the official SI unit is the Pascal [Pa].) Resistance arises from friction, and is proportional to the following expression (Vessel Diameter) 4 Vessel Length Resistance ∝ Viscosity × (2.2) Hence blood flow is slowest in the small vessels of the capillary bed, thus allowing time for the exchange of nutrients and oxygen to surrounding tissue by diffusion through the capillary walls. Approximately 75% of total blood volume is ‘stored’ in the veins which, because of their high capacity, act as reservoirs. Their walls distend and contract in response to the
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